A method to modify the Ultra Wideband（UWB）monopole antenna is proposed to improve its performance. By adding a segment of arc on the ground plane and changing the right angles of the ground plane to fillets，the current distributions of the antenna is changed，the impedance characteristics is improved and the bandwidth is broadened，covering the bandwidth of the ultra wideband that is stipulated by Federal Communication Commission. The antenna is simulated by electromagnetic simulation software Ansoft HFSS，and in the last the radiation patterns of this antenna are given，with the simulation results indicating that the improved antenna is nearly achieved omnidirectional radiation at a lower frequency and also shows the design is practicable and effective.